An endoprosthesis is an artificial device that is placed inside the body, usually within an anatomical lumen. An anatomical lumen is a cavity within the body, typically a tubular organ. Examples of anatomical lumen include without limitation a urinary tract, bile duct, fallopian tube, esophagus, and arterial or venous vasculature. Stents are a type of endoprosthesis which are often used in the treatment of atherosclerotic stenosis in blood vessels. Stenosis refers to a narrowing or constriction of the diameter of an anatomical lumen or orifice. Stents can reinforce the walls of the anatomical lumen and prevent a reoccurrence of stenosis.
Stents may include a scaffold of interconnected structural members, which are referred to as stent struts. Scaffolds were typically made of shape memory metal, such as nickel-titanium alloy, due to demanding design requirements. Requirements include having to be very small (sometimes just a few millimeters in diameter) and having to withstand conditions within a human or animal body for long periods of time. Also, scaffolds are often collapsed or crimped before introduction into to the patient. When in a crimped state, the scaffold may be delivered through tight and tortuous anatomical lumen. When at the intended implantation site, the scaffold is allowed to self-expand or is forcibly expanded.
Scaffolds are often crimped onto catheters which are used to deliver the scaffold to the intended implantation site. A scaffold is required to remain attached to the catheter when traveling through the patient, during which time the scaffold may be exposed to bodily fluids and subjected to significant friction from the walls of anatomical lumen at constrictions and sharp bends. The ability of the scaffold to remain attached to a catheter can be measured according to industry guidelines, such as ASTM F2394-07. In ASTM F2394-07, a securement test performed by a device manufacturer measures the ability of the scaffold remain attached to a catheter balloon. However, a securement test on a freshly crimped scaffold may provide unrealistically favorable results. To avoid this, a pre-test treatment may be performed by the device manufacturer prior to the securement test. The pre-test treatment simulates conditions to which the scaffold might be exposed prior to and/or during clinical use. The pre-test treatment may simulate shipping and handling conditions, pre-soaking, and conditions in anatomical lumen.
Advancements in scaffold design have led to the use of polymers for the core material of the scaffold. Scaffolds made of bioresorbable polymers are designed to degrade after a period of time in order provide certain advantages over metal scaffolds. However, polymer scaffolds usually must satisfy the same basic requirements mentioned above for metal scaffolds. Satisfying those requirements can be challenging since polymers are often not as strong as metals. Generally, a polymer stent strut must be thicker to achieve the same mechanical strength as a metal strut, but there are limits to how much struts may be thickened due to size constraints imposed by anatomical lumen. Also, thicker polymer struts may reduce flexibility of the scaffold, which may make it more difficult to deliver the scaffold through constrictions and sharp bends of anatomical lumen.
Deliverability of the scaffold through constrictions and sharp bends may be enhanced via appropriate design of stent strut patterns, careful selection of polymer materials, and/or special processing designed to increase inherent material strength. Still, there is a continuing need to enhance deliverability of polymer scaffolds through tight and tortuous anatomical lumen.